Offshore pipelines typically run along the seafloor to or from offshore platforms. At the platform, the pipelines run up the legs of the pipelines in a section of pipe called a riser. The junction between the seafloor section of the pipeline and the riser is typically a bend whose radius is usually five times the pipe diameter, typically referred to 5D bends. Additionally, from the point of entry on the platform into the pipeline to the riser there may be three or four other 5D bends to pass.
The design of service tools which are to operate in these circumstances requires that they must be short enough or flexible enough to pass these bends. Flexible can come in the form of a truly flexible service tool, or ball joints spaced along a longer service tool which effectively make flexible.
An added problem occurs when one of these service tools is stranded in the pipeline and it must be recovered. Conventionally a profile called a fishing neck can be added to the end of the tool and a fishing tool can be sent into the pipeline to engage the tool. As these service tools can be in any orientation, the fishing necks are characteristically small and concentric such that they will be near the center of the pipeline.
These fishing tools to “fish” for or engage the stuck service tools must pass the 5D bends on the platform to get into the pipeline. When they approach the service tools, they must be approximately concentric with the pipeline to engage the fishing neck in spite of the effects of gravity or the end loading of the running string. The running string is usually coiled tubing or thin walled steel pipe. This tubing must also pass the 5D bends so are functionally highly stressed and bent at the time the engagement with the fishing neck occurs. The tendency of the end of the coiled tubing is to be adjacent to one side, which makes fishing operations difficult.
An added problem is that whereas the service tools must be a combination of short and flexible enough to pass the 5D bend and the fishing tool must be a combination of short and flexible enough to pass a 5D bend, when the fish is caught and is being retrieved, the combination must be short and/or flexible enough to pass the 5D bend.
Oilfield fishing tools similar to this have been used for almost 100 years, however, are characteristically run into a vertical well bore so centralizing is relatively simple and there are no short radius bends to contend with. Any bend in an oil or gas well will have a radius of greater than 100 feet.
The need for the ability to effectively enter a pipeline travelling past multiple relatively short bends has existed as long as subsea pipelines have existed. The conventional wisdom solution to this problem has been to go to the seafloor, cut the pipeline, and raise the cut end up to the surface such that you will have a straight access into the pipeline. This process is greatly complicated when there is pressure and the potential of environmental pollution.